1. Field of the Invention
The present invention relates to an apparatus and method for magnetic recording and reproducing (hereinafter referred to as "VTR"), or more in particular to a VTR comprising an electro-mechanical converter element constructed of a piezoelectric element or the like on which a magnetic head is mounted, in which the magnetic head is movable along the rotational axis of a rotary cylinder thereby to obtain a special noiseless reproduced image, and the DC component applied to the electro-mechanical converter element is minimized.
2. Description of the prior art
In a method of tracking control of an "8 mm video" as it is commonly called, four frequency pilot signals are recorded in superposition on a video signal, and at the time of reproduction, a tracking error signal is obtained in accordance with the difference in reproduction level between the pilot signals recorded in the tracks on both sides of the track being scanned, and the feed phase of the magnetic tape is controlled by use of this tracking error signal so that the reproduction head may scan on-track along the recording track.
FIG. 1 shows traces of magnetization for recording pilot signals of four frequencies. In FIG. 1, characters A.sub.1, B.sub.1, . . . designate the magnetization traces recorded by the heads A and B respectively, and f.sub.1 to f.sub.4 the pilot signals. The frequency of the pilot signals is given as about 6.5 f.sub.H to 10.5f.sub.H if the frequency of a horizontal sync signal is assumed to be f.sub.H. The frequency difference of the pilot signals recorded in the trarks between the respective tracks is represented by signals having frequencies of f.sub.H and 3f.sub.H, As a result, if the pilot signals reproduced and the pilot signals recorded on the main scanning track are subjected to balanced-modulation to produce signals of f.sub.H and 3f.sub.H, the difference between these signals may be used as a tracking error signal. The method of producing a tracking error signal is well known and therefore will not be described in detail in this specification.
In the system using pilot signals, a tracking error signal is produced over the entire range of the recording track. A rotary magnetic head is mounted on an electro-mechanical converter element including a piezoelectric element or the like displaceably along the direction of rotational axis, that is, along the width of the recording track. In this way, the amount of displacement is controlled by the tracking error signal, thus making possible a control system that can follow any curving of the track.
As a result of making the magnetic head displaceable along the width of the recording track, on the other hand, a reproduced image without noises is obtained at the time of a special reproduction with a tape speed different from that for the recording.
It is, however, difficult to realize a control system for a special reproduction without noises only by feeding back a tracking error signal negatively. At the time of high-speed reproduction, the dynamic range of the control system widens, and therefore the tracking accuracy of a control system with a predetermined gain deteriorates with the increase in the width of the dynamic range. In order to prevent this, the amount of track displacement caused by the difference in tape speed between recording and reproduction is normally applied as a preset voltage to an electro-mechanical converter element, while constituting a control system of closed loop by using a tracking error signal.
FIG. 2 shows a trace of head scanning for triple speed reproduction, and FIG. 3 a preset voltage waveform applied to the electro-mechanical converter element indicated with the track pitch (Tp) as a unit.
In FIG. 2, A.sub.1, B.sub.1, A.sub.2, . . . represent traces of magnetization recorded by respective heads A and B. The arrow 201 indicates the direction in which the magnetic tape is fed, and the arrow 202 the scanning direction of the magnetic heads. If the tape is fed for reproduction at a speed three times higher than that for recording, for example, the traces of the magnetic head relative to the recording track are as shown by 203 to 205. If a reproduced image without noises is to be obtained by frame reproduction under this condition, the scanning is required to leave the traces shown by 206 to 208. The magnetic heads mounted on the electro-mechanical converter element are capable of being displaced along the width of the recording track. It is, therefore, possible to realize the scanning traces 206 to 208 if the electro-mechanical converter element is supplied with zero voltage at a point when the magnetic tape beings to be in contact with the tape, and a potential equivalent to 2Tp at a point where a magnetic head begins to leave the tape.
Waveforms of the applied voltages (preset waveforms) are shown in FIG. 3. FIG. 3(a) shows a head switching signal (hereinafter referred to as the "H.SW signal"). The H.SW signal has the same frequency as the speed of the rotary cylinder and in phase with the rotation of the rotary cylinder. In FIG. 3(a), characters A and B designate the period during which the heads A and B are in contact with the magnetic tape respectively. FIG. 3(b) shows a preset voltage waveform applied to the electro-mechanical converter element, as converted into a track pitch. In FIG. 3(b), the displacement in the same direction as the feed of the magnetic tape is shown as a positive displacement. If the preset voltage shown in FIG. 3(b) is applied to the electro-mechanical converter element, the head scanning shown in 206 to 208 of FIG. 2 is realized.
In the case where a piezoelectric element is used as the electro-mechanical converter element, it is desirable to keep the average DC level of the applied voltage at zero. Protracted application of a DC voltage to the piezoelectric element would deteriorate the performance such as sensitivity.
Even for the electro-mechanical converter element including other than a piezoelectric element, it is desirable to keep the average DC level of the applied voltage to zero. In the method where only the displacement in positive direction is used as shown in FIG. 3(b), for instance the dynamic range is considerably reduced as compared with the method in which the displacements in both positive and negative directions are used.
A method for reducing the average DC voltage to zero is disclosed previously in the Japanese Laid-Open Patent Publication No. 61-74128. In this conventional method, the center potential of the preset voltage is sampled out at intervals of j, and the center potential is reduced to zero. This method is effective for the triple-speed reproduction as shown in FIG. 3, but not always so for N-fold speed reproduction. This point will be explained below.
FIG. 4 is a diagram showing the head scanning traces for 1/4-fold speed reproduction, and FIG. 5 a preset waveform required for the same.
In FIG. 4, the arrow 401 designates the direction in which the tape is fed, and the arrow 402 the scanning direction of the magnetic head. Normally, the field reproduction is used for low-speed reproduction of one-fold or less. If the frame instead of field reproduction is used, two images are produced alternately at intervals of 1/60 seconds, so that a moving image is blurred in reproduction, thus deteriorating the image quality in the NTSC system.
For the purpose of field reproduction, the head scanning traces 403 in FIG. 4 are adapted for scanning the track B.sub.1, and the head scanning traces 404 the track B.sub.2. A preset waveform required for this purpose is shown in FIG. 5(b). FIG. 5(a) shows an H.SW signal.
If the conventional method mentioned above is applied to the 1/4-fold speed reproduction shown in FIG. 5, the number of samplings is required to be 8. After j is set to 8, it is still necessary to decide whether the preset waveform sampled is represented by 501 or 502 shown in FIG. 5. Further, since the speed of special reproduction is not limited to 1/4 times, the value cannot be limited to 8. Specifically, the method of reducing the center potential of the preset waveform sampled at intervals of j to zero cannot be easily applied to all given N-fold speed.
If the average DC level off the voltage applied to an electro-mechanical converter element is to be reduced to zero, it is necessary to take appropriate measure on the voltage waveform applied to each head for each frame.
FIG. 6 shows conventional voltage waveforms applied to each electro-mechanical converter element at the time of triple-speed reproduction.
In this drawing, character a designates an H.SW signal. In FIG. 6, the head A is contact with the magnetic tape during the "high" period, and the head B during the "low" period. FIG. 6(b) shows a voltage waveform for displacing the head A, and FIG. 6(c) a voltage waveform for displacing the head B. The voltage waveforms 601 and 602 are supplied to each electro-mechanical converter element to displace the respective heads. The applied voltage is positive in the direction of feeding the magnetic tape, and the voltage level is converted to track pitch (Tp). Take the head A as an example. At the time point when the magnetic head begins to be in contact with the tape, a voltage equivalent to -1Tp is applied, while at the time point when the magnetic head leaves the tape, a voltage equivalent to +1Tp is applied. In the meantime, the potential is changed in linear fashion. The voltage may take a given waveform during the time when the magnetic head is not in contact with the tape. In a normal method that has been used so far, a saw-toothed waveform shown by 603 in FIG. 6(b) is generated first, and this waveform is passed through a low-pass filter to obtain a waveform designated by 601, which in turn is applied to an electro-mechanical converter element.
This voltage waveform, however, has different sizes of areas of the parts designated by 604 and 605 as shown in FIG. 6(c). Specifically, the average DC potential is not zero, thereby posing the problem mentioned above.